The internal environment of animals provides attractive conditions for the growth of bacteria, viruses, and other organisms. Although some of these organisms can live symbiotically within animals, within animals, many either cause destruction of cells or produce toxic chemicals. To protect against these foreign invaders humans have a three level defense system.
The internal environment of animals provides attractive conditions for the growth of bacteria, viruses, and other organisms. Although some of these organisms can live symbiotically within animals, within animals, many either cause destruction of cells or produce toxic chemicals. To protect against these foreign invaders humans have a three level defense system.